User equipment, feedback control method, and retransmission control method

ABSTRACT

User equipment to be used as receiving user equipment in a mobile communication system supporting D2D communication, the user equipment includes a feedback unit that receives a D2D signal from transmitting user equipment, and that transmits, to the transmitting user equipment, a feedback signal with respect to the D2D signal by using a predetermined resource; and a receiver that receives a retransmission D2D signal transmitted from the transmitting user equipment based on the feedback signal.

TECHNICAL FIELD

The present invention relates to D2D communication (user equipment touser equipment communication), and particularly relates to a techniquefor user equipment UE that receives a D2D signal to transmit a feedbacksignal in the D2D communication. Note that the D2D communication mayalso be referred to as side link (sidelink) communication.

BACKGROUND ART

In the current mobile communication system, such as LTE, it is commonthat communication between user equipment UE and user equipment UE isperformed through a base station eNB by performing communication betweenthe user equipment UE and the base station eNB; and, in recent years,various techniques have been proposed for the D2D communication (whichis referred to as “D2D,” hereinafter) in which communication is directlyperformed between user equipment UE and user equipment UE.

In particular, for D2D in LTE, the following have been proposed:“Communication (communication)” for performing data communication, suchas a push-type call, between user equipment UE and user equipment UE;and “Discovery (detection)” (see Non-Patent Document 1) for causingreceiving user equipment to detect user equipment Transmitting UE bytransmitting, from the user equipment UE, a discovery signal (discoverysignal) including its own ID, an application ID, etc.

For the D2D to be specified in LTE, it has been proposed that each userequipment UE utilizes a part of an uplink resource that has already beenspecified as a resource for uplink signal transmission from the userequipment UE to a base station eNB. Additionally, it has been proposedthat a base station eNB assists for allocating a resource to be used inD2D. An overview of the currently proposed resource allocation for theD2D communication in LTE is described below (see Non-Patent Document 1).

For the “Discovery,” as illustrated in FIG. 1A, a resource pool ofDiscovery signals is reserved for each Discovery period; and userequipment UE transmits a Discovery signal within the resource pool. Morespecifically, there are Type 1, Type 2a, and Type 2b. For Type 1, userequipment UE autonomously selects a transmission resource from aresource pool. For Type 2a, a specific resource in a resource pool is tobe dynamically allocated by a (E)PDCCH. For Type 2B, a semi-persistentresource is to be allocated by higher-layer signaling (e.g., a RRCsignal).

For the “Communication,” as illustrated in FIG. 1B, it has been studiedto periodically reserve a resource pool for SA/Data transmission. SA isan abbreviation of Scheduling Assignment; and transmitting UE reports aresource for Data transmission to a receiving side with a resourceselected from the SA resource pool, and transmits Data with the resourcefor Data transmission. This signal for reporting the resource may bereferred to as SA. For the “Communication,” more specifically, there areMode 1 and Mode 2. For Mode 1, a resource is to be dynamically allocatedby a (E)PDCCH to be transmitted from a base station eNB to userequipment UE. Furthermore, for allocation of a (E)PDCCH, asemi-persistent resource allocation (SPS: Semi-persistent scheduling)has been proposed. For Mode 2, user equipment UE autonomously selects atransmission resource from the SA resource pool. Note that, for theresource pool, that of reported by a SIB, or a predefined one is to beused.

FIG. 1C specifically illustrates an example where a D2D resource pool ismultiplexed with WAN resources by FDM/TDM. As illustrated in a signalconfiguration of FIG. 1C, signal configurations based on the PUSCH arerespectively adopted for SA and Data.

PRIOR ART DOCUMENT Non-Patent Document

Non-Patent Document 1: 3GPP TR 36.843 V12.0.1 (2014 March)

SUMMARY OF INVENTION Problem to be Solved by the Invention

For the D2D communication in Rel-12 of LTE, broadcast (Broadcast) issupported, and, for group cast (Group-cast) and unicast (Unicast), onlyL1 ID and MAC source/destination ID are specified; and, in Rel-13,detailed functions of Group-cast and Unicast are to be specified.

Furthermore, for the D2D of Rel-12, only Repetition is supported, whichis for repeatedly transmitting SA/Data etc.; and retransmission based onHARQ feedback and CSI feedback are not supported. Accordingly,retransmission for a case where data is not delivered, etc., is to beperformed by a higher layer, for example.

However, especially for the Unicast, the control depending on theretransmission by the higher layer may not be efficient from aperspective of transmission latency/transmission overhead; and, withoutfeedback, enhancement of frequency utilization efficiency by Linkadaptation may not be properly achieved. Consequently, an achievabledata rate is low, and it is not suitable for broadband communication.

The present invention is achieved in view of the above-described point,and an object is to provide a technique for allowing feedback andretransmission to be performed between user equipment and user equipmentin a mobile communication system supporting the D2D communication.

Means for Solving the Problem

According to an embodiment of the present invention, there is provideduser equipment to be used as receiving user equipment in a mobilecommunication system supporting D2D communication, the user equipmentincluding a feedback unit that receives a D2D signal from transmittinguser equipment, and that transmits, to the transmitting UE, a feedbacksignal with respect to the D2D signal by using a predetermined resource;and a receiver that receives a retransmission D2D signal transmittedfrom the transmitting UE based on the feedback signal.

Furthermore, according to an embodiment of the present invention, thereis provided user equipment to be used as transmitting UE in a mobilecommunication system supporting D2D communication, the user equipmentincluding a receiver that receives, from receiving user equipment, afeedback signal with respect to a D2D signal transmitted to thereceiving UE; and a transmitter that transmits, to the receiving UE, aretransmission D2D signal with respect to the D2D signal, based on thefeedback signal.

Furthermore, according to an embodiment of the present invention, thereis provided a feedback control method to be executed by user equipmentthat is used as receiving user equipment in a mobile communicationsystem supporting D2D communication, the feedback method including afeedback step of receiving a D2D signal from transmitting UE, andtransmitting, to the transmitting UE, a feedback signal with respect tothe D2D signal by using a predetermined resource; and a reception stepof receiving a retransmission D2D signal transmitted from thetransmitting UE, based on the feedback signal.

Furthermore, according to an embodiment of the present invention, thereis provided a retransmission control method to be executed by userequipment that is used as transmitting UE in a mobile communicationsystem supporting D2D communication, the retransmission control methodincluding a reception step of receiving, from receiving user equipment,a feedback signal with respect to a D2D signal transmitted to thereceiving UE; and a transmission step of transmitting, to the receivingUE, a retransmission D2D signal with respect to the D2D signal, based onthe feedback signal.

Advantage of the Invention

According to embodiments of the present invention, there is provided atechnique for allowing feedback and retransmission to be performedbetween user equipment and user equipment in a mobile communicationsystem supporting the D2D communication.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram for illustrating D2D communication;

FIG. 1B is a diagram for illustrating the D2D communication;

FIG. 1C is a diagram for illustrating the D2D communication;

FIG. 2 is a configuration diagram of a system according to an embodimentof the present invention;

FIG. 3A is a diagram illustrating an example of a channel for feedback;

FIG. 3B is a diagram illustrating an example of a channel for feedback;

FIG. 4 is a diagram for illustrating feedback SCI;

FIG. 5 is a diagram illustrating an example 1 of feedback by Data;

FIG. 6 is a diagram illustrating an example 2 of the feedback by Data;

FIG. 7 is a diagram illustrating an example 3 of the feedback by Data;

FIG. 8 is a diagram illustrating an example of allocation of a feedbackresource;

FIG. 9 is a diagram illustrating an example of a symbol mappingconfiguration based on PUCCH format 1;

FIG. 10 is a diagram illustrating an example of a symbol mappingconfiguration based on PUCCH format 3;

FIG. 11 is a diagram illustrating details of DM-RS in a PUCCH-basedchannel configuration;

FIG. 12A is a diagram illustrating bundling of ACK/NACK feedback;

FIG. 12B is a diagram illustrating the bundling of the ACK/NACKfeedback;

FIG. 12C is a diagram illustrating the bundling of the ACK/NACKfeedback;

FIG. 13 is a diagram illustrating an example of mapping of feedbackinformation to a channel;

FIG. 14 is a diagram illustrating an example of scheduling SCI;

FIG. 15 is a diagram for illustrating a mixed scheduling ofretransmission/new transmission;

FIG. 16 is a configuration diagram of user equipment UE according to afirst embodiment;

FIG. 17 is a diagram illustrating an example 2-1 of a processingprocedure according to a second embodiment;

FIG. 18 is a diagram illustrating an example 2-2 of the processingprocedure according to the second embodiment;

FIG. 19 is a diagram illustrating an example 2-3 of the processingprocedure according to the second embodiment;

FIG. 20 is a configuration diagram of the user equipment UE according tothe second embodiment;

FIG. 21 is a configuration diagram of a base station eNB according tothe second embodiment;

FIG. 22 is a diagram illustrating an example 3-1 of a processingprocedure according to a third embodiment;

FIG. 23 is a diagram illustrating an example 3-2 of the processingprocedure according to the third embodiment;

FIG. 24 is a configuration diagram of the user equipment UE according tothe third embodiment;

FIG. 25 is a configuration diagram of the base station eNB according tothe third embodiment;

FIG. 26 is a HW configuration diagram of the user equipment UE; and

FIG. 27 is a HW configuration diagram of the base station eNB.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are described below by referring tothe drawings. The embodiments described below are merely examples, andembodiments in which the present invention is applied are not limited tothe embodiments described below. For example, as a mobile communicationsystem according to the embodiments, a system based on a schemeconforming to LTE is assumed; however, the present invention is notlimited to LTE, and is also applicable to another scheme. Furthermore,in the following, feedback with respect to Data in Communication ismainly described; however, the feedback technique according to thepresent invention is not limited to Data, and is generally applicable toD2D signals. Furthermore, in the present specification and the scope ofthe claims, “LTE” is used in a broad sense including communicationschemes corresponding to 3GPP Rel-12, 13, or after that.

In the following description, retransmission based on feedback, such asNACK, is denoted as “transmission,” and repetition of transmission isdenoted as “Repetition.” Furthermore, as described below, controlinformation transmitted in a SA resource pool can be referred to as SCI(Sidelink Control Information); however, the term “SCI” is mainly usedfor “feedback SCI” and “scheduling SCI” that is specifically describedas a SA provided with a new format, and the term “SA” is mainly used fora SA, for convenience. However, even if it is described as “SA,” it isnot limited to existing SAs.

Furthermore, ACK/NACK, CSI, etc. are collectively referred to asfeedback signals. The meaning of a feedback signal also includes a SAfor cases where ACK/NACK, CSI, etc. are transmitted by Data.

System Configuration

FIG. 2 is a diagram illustrating an example of a configuration of amobile communication system according to an embodiment (which is commonamong embodiments) of the present invention. As illustrated in FIG. 2,the communication system according to the embodiment is a cellularcommunication system in which user equipment UE 1 and user equipment UE2 are under the control of a base station eNB. Each of the userequipment UE 1 and UE 2 is provided with a D2D communication function,so that D2D communication can be performed between the user equipment UE1 and UE 2. Furthermore, the user equipment UE 1 and the user equipmentUE 2 are respectively capable of performing usual cellular communicationwith the base station eNB; and are respectively capable of receivingresource allocation for the D2D communication from the base station eNB.

FIG. 2 illustrates that the user equipment UE 1 and the user equipmentUE 2 are within coverage of the base station eNB; however, this is anexample, and the present invention can be implemented even if userequipment UE is outside coverage of a base station eNB. In thefollowing, the user equipment UE 1 and the user equipment UE 2 arecollectively denoted as user equipment UEs or UEs. Furthermore, userequipment UE at a side of transmitting SA/Data (the side of receivingfeedback) is denoted as a transmitting UE; and user equipment UE at aside of receiving it (the side of transmitting the feedback) is denotedas a receiving UE.

In the following, a first embodiment, a second embodiment, and a thirdembodiment are described. In the first embodiment, a technique forperforming feedback of ACK/NACK and/or reception quality (CSI: ChannelState Information) with respect to transmission Data is described. Inthe second embodiment, a technique for retransmission at a time of Mode1 resource allocation is described. In the third embodiment, feedbacksetting and switching of a feedback type are described.

First Embodiment

Now, the first embodiment is described. In the following, resourceconfiguration examples 1 through 3 for feedback are described.

Resource Configuration Example 1 for Feedback

A resource group dedicated for feedback (which is referred to aschannels for feedback) is defined for an example 1 of a resourceconfiguration, as a resource for transmitting a feedback signal from thereceiving UE to the transmitting UE in the D2D communication; and thefeedback channels are to be used. By using the feedback channels,feedback with low latency can be achieved.

FIG. 3A and FIG. 3B illustrate examples of resource configurations ofthe feedback channels. In the example of FIG. 3A, the hatched resourcepool is defined as the feedback channels. In FIG. 3A, as an example, oneresource pool of the feedback channels is indicated as “Response.”

As illustrated in FIG. 3A, in this example, the resource pool of thefeedback channels is arranged after a resource pool for Datatransmission (temporally later) and prior to a SA resource pool(temporally before). For example, after the transmitting UE transmitsSA/Data to the receiving UE using resources in the resource poolsindicated by A and B, the receiving UE transmits a feedback signal tothe transmitting UE by using a resource in the resource pool indicatedby C, which is a feedback resource up to the subsequent SA resourcepool.

Note that the feedback channels (the resource pool for feedback) may beconfigured between the start of DFN/SFN (a system frame number) and asubframe specified by a SA offset indicator. By such a configuration, anSA/Data transmission resource pool that is compatible with a Rel-12 UEcan be defined.

Furthermore, a resource for a feedback channel may be configured asillustrated in FIG. 3B. In the example of FIG. 3B, the SA resource poolis time-divided, the first half is used as feedback channels, and thelatter half is used for SA transmission.

Furthermore, the resource pool for feedback may be configured, forexample, by a base station eNB by using system information (SIB) foreach UE within coverage; and, as the contents of the configuration atthat time, there are, for example, assignment of resource blocks (RBs)of the resource pool, subframes, periods, and so forth. Furthermore, theSA resource pool/the Data resource pool/the feedback resource pool forCommunication may be mapped to be one-to-one; and the Discovery resourcepool/the feedback resource pool for Discovery may be mapped to beone-to-one.

Furthermore, for outside the coverage, a feedback resource pool may beconfigured in the UE in advance; and the UE may use the feedbackresource pool when it is outside the coverage.

Note that, in the resource configuration example 1, the base station eNBmay explicitly allocate, to the UE, a resource (e.g., a time/frequencyposition) for the receiving UE to transmit a feedback signal by UEdedicated signaling (examples: PDCCH/SCH, RRC signaling, etc.), forexample. Especially, as described later, when the base station eNB is toreceive a feedback signal from the UE, it is desirable to explicitlyallocate the resource in this manner.

Furthermore, the transmitting UE may allocate, to the receiving UE, aresource (a time/frequency position) for transmitting a feedback signalby the receiving UE. This allocation may be explicitly performed, forexample, by using the SA, or may be implicitly performed.

As an example of implicit allocation, a correspondence (mapping) betweena resource position of SA/Data and a resource position for feedback maybe defined in advance; and the receiving UE may determine a feedbackresource based on the resource position of the received SA/Data, and maytransmit a feedback signal by the feedback resource. Furthermore, acorrespondence (mapping) between a resource position of Discovery and aposition of a feedback resource may be defined in advance; and thereceiving UE may determine a feedback resource based on a resourceposition of the received Discovery, and may transmit a feedback signalby the feedback resource.

Furthermore, in addition to above, the receiving UE may, for example,voluntarily (e.g., randomly) select a resource in a configured feedbackresource pool to transmit a feedback signal.

Resource Configuration Example 2 for Feedback

In the resource configuration example 2 for feedback, a resource in theSA resource pool is used as a resource for the receiving UE to transmita feedback signal to the UE at the transmitting side. More specifically,new SCI is defined, and the SCI is transmitted as a feedback signal.Here, the SCI is an abbreviation of Sidelink Control Information, whichmeans control information in the SA resource pool. The signal of the SA,which is described in the part of the background art, is an example ofthe SCI; and, since scheduling is performed, it can be referred to asthe scheduling SCI. Note that “Sidelink” means the D2D communication.

The SCI used for feedback is referred to as feedback SCI (Feedback SCI).The receiving UE may only report a feedback signal, such asACK/NACK/SCI, etc., to the transmitting UE by using the feedback SCI; ormay report, in addition to the feedback signal, Index information forexplicitly reporting a correspondence (the feedback is for which Data)to initial transmission Data (Data which is not for retransmission),which becomes the target of the feedback.

In this manner, by transmitting feedback signals by using resources inthe SA resource pool, the HARQ control can be implemented withoutdefining a new resource pool, so that an increase in overhead due tofeedback can be avoided.

FIG. 4 illustrates an example of mapping of the feedback SCI. Asillustrated in FIG. 4, in this example, feedback SCI (a feedbackmessage) for Data corresponding to SA transmitted from the transmittingUE in the SA resource pool indicated by A is transmitted from thereceiving UE with a resource in the SA resource pool indicated by B.

In FIG. 4, as an example, it is indicated that, in the SA resource pool,the SA resource position and the feedback SCI resource position have apredetermined correspondence. Namely, in this example, the receiving UEcan determine the resource for the feedback SCI based on the position ofthe resource with which the SA is received; and can transmit thefeedback SCI by using the resource.

In the resource configuration example 2, control may be performed suchthat the transmitting UE that has transmitted the SA in a SA resourcepool does not perform new scheduling for the next SA resource pool (doesnot transmit SA). The reason is that, in the D2D, there is the Halfduplex restriction that the UE may not simultaneously performtransmission and reception (in the same subframe), so that, if SAtransmission is performed in a certain subframe in the next SA resourcepool, the feedback SCI, which may be received in the subframe, may notbe successfully received.

Furthermore, the transmitting UE that has transmitted the SA in acertain SA resource pool may detect only the feedback SCI in the next SAresource pool. In this manner, for a case where reception of thefeedback SCI is expected, by only performing detection of the feedbackSCI, the number of times of SCI blind detection (the number of times ofsearch) can be reduced, and the feedback SCI can be efficientlydetected.

Furthermore, in order to avoid collision between feedback SCI and SA(scheduling SCI), in Mode 2 (the mode in which resource selection isautonomously performed), the receiving UE that detects the SA (thescheduling SCI) requesting feedback may exclude the feedback SCItransmission resource in the next SA cycle from transmission resourcecandidates for the SA (the scheduling SCI).

Here, if, in the UE, the SA (the scheduling SCI) collides with thetransmission/reception subframe of the feedback SCI, one of them may beprioritized. For example, transmission and reception may be formed inthe order of precedence represented by “feedback SCI reception>feedbackSCI transmission>scheduling SCI transmission>scheduling SCI reception,”in descending order of priority. For example, if, in a certain UE,feedback SCI reception and scheduling SCI transmission are in the samesubframe, the feedback SCI reception is performed in the subframe. Theabove-described order is an order from the perspective thatretransmission likely occurs between UEs in the near future; and it isan example. In this manner, by prioritizing the operation in whichretransmission may occur, overheads and latency can be reduced.

Resource Configuration Example 3 for Feedback

In the resource configuration example 3 for feedback, a feedback signalis to be transmitted by using Data of the D2D at the receiving UE.Namely, ACK/NACK and CSI are transmitted as Data. In this manner, byusing Data for feedback, many ACK/NACK bits can be fed back.

More specific three examples (example 1 through example 3) are describedby referring to FIG. 5 through FIG. 7.

In example 1 (FIG. 5), the receiving UE processes the feedback signal asnormal Data. Namely, in FIG. 5, SA/Data is transmitted from thetransmitting UE with the resources indicated by A and B. Then, thereceiving UE transmits the SA for transmitting the feedback signal asData (which includes the resource position of Data as the feedbacksignal) with the resource C; and transmits the feedback signal with theresource D. Note that, in FIG. 5 (the same applies to other figures), itis indicated that Repetition is performed twice for the SA, andRepetition is performed four times for Data.

In example 2 (FIG. 6), the receiving UE transmits a feedback signal forthe received Data as Data (with the Data resource) to the transmittingUE within the same cycle as the SA/Data transmitted from thetransmitting UE. In FIG. 6, it is indicated that the resource for thereceived Data is associated with the resource for the feedback signalwith respect to the Data.

Namely, the correspondence (mapping) is defined in advance between theresource of Data indicated by SA transmitted from the transmitting UEand the resource to be used for feedback transmission. This may beconfigured in the UE by signaling from the base station eNB to the UE;or may be configured in advance in the UE. Furthermore, a plurality ofcorrespondence relationships (mappings) may be defined, and one of themmay be used. Based on the correspondence, the receiving UE can transmitthe feedback signal as Data.

Note that information indicating the Data resource position for feedbackmay be included in T-RPT (which is included in the SA) specifying apattern etc. of Repetition of Data; and the receiving UE may determinethe Data resource for the feedback based on the information.

Furthermore, it may be configured such that one feedback resource isallocated to K (K is an integer greater than or equal to 1) times ofData transmissions (initial and Repetition), and K is configurable. Theexample of FIG. 6 is the example where K=1 (Data and the feedback isone-to-one). Furthermore, the frequency domain resource of the feedbacksignal may be the same as or different from the resource allocation bythe SA from the transmitting UE.

In example 3 (FIG. 7), the transmitting UE reports, to the receiving UE,the Data resource position for the feedback transmission by using theSA. In the example of FIG. 7, the transmitting UE transmits the SA (SA2)for allocating the feedback resource in the same SA cycle as the SA(SA1) for Data transmission from the transmitting UE.

Namely, in FIG. 7, the receiving UE receives Data (indicated by B) withthe resource specified in the SA1 (indicated by A) for Data; andtransmits, to the transmitting UE, the feedback signal (indicated by D)with the resource specified in the SA2 (indicated by C) for thefeedback.

For the SA (the scheduling SCI) specifying the resource for thefeedback, a new field for specifying the resource for the feedback maybe formed in the existing SA. Furthermore, the resources fortransmitting the SA1 and the SA2 may be different subframes, or may beconsecutive frequency positions within the same subframe.

Furthermore, the resources of the Data/feedback signals specified theSA1/SA2 may be associated, or may be independent from each other.

Details of the Above-Described Example 3

In transmission of the normal SA, the SA is transmitted by the Datatransmission side; however, in the above-described example 3, even ifthe transmitting UE transmits the SA (SA2), Data corresponding to the SAis not to be transmitted; and Data (feedback) corresponding to the SA(SA2) is transmitted by the side receiving the SA (SA2). In the normaloperation, the transmitting UE reports, to the receiving UE,transmission timing (TA) of Data by the SA; however, in the operation ofthe above-described example 3, the transmission timing is not reported.Namely, the receiving UE may not report, to the transmitting UE, thetransmission timing of Data to be transmitted as feedback.

In view of the above-described point, in the embodiment, the receivingUE uses the transmission timing (TA) reported by the SA (SA2 in FIG. 7),which is transmitted from the transmitting UE, as the transmissiontiming of Data to be transmitting as the feedback signal.

By using such a method, timing uncertainty at the time of receiving thefeedback signal can be minimized for the transmitting UE. Furthermore,the transmitting UE may report, to the receiving UE by the TA, whetherit knows the transmission timing for the receiving UE, which transmitsthe feedback.

Furthermore, the receiving UE may transmit Data (feedback signal) basedon the timing information of the receiving UE itself without using theTA bits reported from the transmitting UE.

The timing information here is, for example, own UL transmission timing,DL reception timing, etc. Depending on the state of the RRC, forexample, in the RRC CONNECTED mode, the UL timing may be used; and inthe IDLE mode, the DL timing may be used. In this manner, bytransmitting Data (feedback signal) without using TA, the overhead forthe TA reporting can be reduced. Furthermore, the corresponding bits canbe used for reporting any other feedback related information.

Here, in the normal SA transmission, it can be considered to include theID of the Data receiving side, as an ID. In this case, for the case ofSA2 of example 3, since Data (feedback signal) is received by thetransmitting UE that transmits the SA (SA2), it can be considered toinclude, as the L1 ID, the transmitting UE's ID in the SA2. In thiscase, it can be considered that, in the receiving UE that expects the SAincluding the own ID, the SA2 may not be detected.

In view of these points, for SA for a special purpose, such as the SA2of the example 3, a format that is different from that of normal SA maybe used or information (Contents) indicating that it is for a specialpurpose may be included; and, in response to detecting the format of thespecial SA or the Contents, the receiving UE may receive the SA (SA2)regardless of the L1 ID, and may determine whether Data (feedbacksignal) transmission is required. Alternatively, as the L1 ID, a UnicastID may be used, which is to be detected by the receiving UE.

Note that the above-described resource configuration examples 1, 2, and3 may be properly switched to be applied. As a method of switching, thebase station eNB may instruct the UE, through higher layer signaling,which method is to be used; or the UE may autonomously switch the methoddepending on the coverage state, such as within coverage/outsidecoverage, or a mode of the D2D (Mode 1, Mode 2). By configuring thesystem to be able to be switched as described above, efficiency of thefeedback can be optimized depending on the degree of freedom andcertainty of resource allocation.

Example of Feedback Resource Allocation

An example of a method of allocating a feedback resource according tothe embodiment is described by referring to FIG. 8, which can be appliedto the above-described resource configuration example 1, the resourceconfiguration example 2, the example 1 and example 2 of the resourceconfiguration example 3, and so forth.

In this example, correspondence is defined between the resource fortransmitting the SA (receiving, if it is viewed from the receiving UE)and the resource for transmitting the feedback signal (which includesthe SA for the Data for transmitting the feedback information), and thefeedback transmission resource is implicitly reported to the receivingUE by the SA transmission resource from the transmitting UE. Namely, thereceiving UE can determine the feedback transmission resource based onthe reception resource of the SA received from the transmitting side;and can use it for transmitting the feedback signal.

For example, as illustrated in (a) of FIG. 8, a resource may bedetermined, so that the feedback signal corresponding to the SAtransmitted in a certain subframe is to be transmitted in a subframehaving the same number as the certain subframe. Furthermore, forexample, when SA-A, SA-B, and SA-C are transmitted from one or more UEsto the receiving UE in a subframe X, the receiving UE may transmit, in asubframe Y, feedback signals respectively corresponding to these SAs.Namely, feedback transmission may be performed in the same subframe forSA transmissions in the same subframe. In this manner, by restricting,for the transmitting UE, the subframe in which the feedback signals areto be received, the effect of the Half duplex can be avoided.

Furthermore, the transmitting UE may include, in the SA, a parameterused for determining the transmission resource of the SA (example: theparameter S as the SA resource index) to be transmitted to the receivingUE; and the receiving UE may determine a resource for transmitting thefeedback signal by using the parameter.

Furthermore, a plurality of feedback transmission resource candidatesmay be defined for each SA transmission resource; and the receiving UEmay switch a resource to be actually used, depending on the Contents(e.g., ARI: ACK/NACK resource indicator) included in the SA. In theexample of (b) of FIG. 8, a case is illustrated where CDM (code divisionmultiplexing) is to be performed. In the example of (b) of FIG. 8, twofeedback transmission resource candidates are defined for each SAtransmission resource; and the receiving UE selects one resource toperform transmission of the feedback signal, depending on the Contentsof the SA.

By adopting the above-described method, the effect of the Half duplexcan be avoided. Furthermore, Data and feedback can be dynamicallyassociated; and reduction of the overheads of the feedback resources andrandomization of collisions among the feedback resources can beachieved.

Note that the above-described feedback transmission resource candidatemay be determined based on the transmission resource of the SA; or maybe determined by the higher layer signaling from the base station eNB.Furthermore, information on the correspondence between the resource ofthe SA and the resource of the feedback signal may be preset in each UE;may be reported from the base station eNB to each UE by the higher layersignaling; or may be shared by signaling between UEs.

Furthermore, the receiving UE may determine the resource size fortransmitting the feedback signal depending on the number of thetransmission subframes of Data transmitted from the transmitting UE.

For example, the receiving UE can determined the resource size of thefeedback signal based on the number of Data subframes of the D2D thatcan be used during the Data scheduling period (e.g., from the end of theSA resource pool to the start of the subsequent SA resource pool).Furthermore, the receiving UE may determine the resource size of thefeedback signal based on the number of the subframes used for actualData transmission, which is reported by T-PRT, etc. included in the SA.As a specific determination method, for example, a function may bedefined such that the resource size of the feedback signal becomesgreater, as the above-described number of the subframes of Dataincreases; and the resource size of the feedback signal may bedetermined by using the function. By determining the resource size ofthe feedback signal in this manner, an appropriate feedback amount canbe selected depending on the amount of the transmission data.

Furthermore, in order to reduce the retransmission latency, a SA cycleand the number of SA resource pool subframes smaller than those ofRel-12 may be configured. In this manner, for example, it is consideredthat there is a case where a subframe capable of Data transmissionstarts prior to the last subframe of the SA resource pool; however, inthis case, the scheduling by the SA in the SA resource pool may scheduleData in the subsequent SA cycle.

Feedback Signal Configuration

As described in the “resource configuration example 1 for feedback,”when a feedback channel (a resource) is to be newly defined, aconfiguration may be adopted which is based on the PUSCH similar to theSA, or a PUCCH format may be used, as the signal configuration in thechannel.

FIG. 9 shows examples (design examples 1 through 3) of the signalconfiguration based on the PUCCH format 1: and FIG. 10 shows examples(design examples 1 through 4) of the signal configuration based on thePUCCH format 3. Note that the signal configurations shown in FIG. 9 andFIG. 10 are examples, and it is not limited to these. In FIG. 9 and FIG.10, feedback information is mapped onto the Data symbol.

In the PUCCH format of the D2D, the last symbol of one subframe ispunctured (no transmission). In the design examples 1 and 2 of FIG. 9,and in the design examples 1, 2, and 3 of FIG. 10, considering the lastsymbol of the subframe to be punctured, different symbol mappings areused between the slots. In this manner, if different configurations areadopted between the slots, it is possible to maximize the use of thesymbols of the first slot.

Furthermore, in the design example 3 of the FIG. 9 and the designexample 4 of FIG. 10, the same symbol mappings are used between theslots by additionally puncturing the last symbol of the first slot. Inthis manner, if common configurations are adopted between the slots,reception complexity (Complexity) can be reduced.

Note that, in the examples of FIG. 9 and FIG. 10, frequency hopping isperformed between slots; however, the two slots (one subframe) may betransmitted with the same frequency resource without performingfrequency hopping between the slots.

In the present embodiment, multiple feedback signals can be codedivision multiplexed (CDM) in the same time and frequency resource, andthereby the time and frequency resource can be effectively utilized.Accordingly, in the embodiment, DM-RSs, which are indicated as “RSsymbol” in FIG. 9 and FIG. 10, may be made orthogonal.

FIG. 11 shows a configuration example of the DM-RS in a PUCCH-basedfeedback signal. The Group hopping, the sequence hopping, and the deltashift, which are shown in the frame A of FIG. 11 and used for generatinga DM-RS base sequence (DM-RS base sequence), may be fixed; or may be thesame as those of Data.

Furthermore, for example, by using a fixed value or a value that iscommon among feedback signals multiplexed in the same resource in theCell ID field used for sequence generation, orthogonal multiplexing ofthe DM-RSs by CS/OCC (Cyclic shift/Orthogonal Cover Code) (B in FIG. 11)can be achieved.

Furthermore, when the feedback signals are code division multiplexed,DM-RS orthogonalization among the feedback signals multiplexed in thesame time/frequency resource, which correspond to different SCIs, can beachieved by the CS/OCC.

Note that, as in the first example shown in B of FIG. 11, if the CS/OCCis determined based on the L1 ID, collision may occur among the feedbacksignals; however, if the CS/OCC is determined based on the ID/indexrelated to HARQ, and/or the SCI resource index, the collision can becompletely avoided. Here, the SCI in the SCI resource index is the SCIfor resource allocation of Data, which is transmitted from thetransmitting UE, and which is the target of the feedback.

Feedback Content

In the embodiment, as the contents of the feedback signal transmittedfrom the receiving UE to the transmitting UE, there are the ACK/NACK andthe CSI. Note that these are examples, and any other information may betransmitted as the feedback signal. An example of a specifictransmission method of the ACK/NACK and the CSI is described below.Furthermore, the examples described below can be applied to all thefeedback transmission methods (e.g., the resource configuration examples1, 2, and 3), except as indicated otherwise.

Feedback Content Example 1: ACK/NACK

First, the ACK/NACK is described. In the embodiment, the receiving UEdetermines ACK/NACK for each MAC PDU received from the transmitting UE;and reports the ACK/NACK to the transmitting UE, as the feedback signal.However, as described later, the receiving US may group the ACK/NACK tobundle it (Bundling), so that the ACK/NACK may be fed back with fewerresources.

Furthermore, in the feedback signal to be reported from the receiving UEto the transmitting UE, the number of the MAC PDUs to be fed back,and/or the number of the ACKs/NACKs may be included to be reported. As aresult, the transmitting UE (the UE receiving the feedback) canrecognize the number of the MAC PDUs, and/or the number of theACKs/NACKs to be received as the feedback, so that errors in receivingthe feedback can be avoided. Additionally, a confirmation bit stream maybe included in the feedback signal to be reported from the receiving UEto the transmitting UE. The confirmation bit stream is a stream to betransmitted as the feedback that can be associated with (a sequence of)MAC PDUs; and it can be generated from the T-RPT etc. in the SA, forexample. Errors in receiving the feedback can also be avoided by thismethod.

Feedback Content Example 2: CSI

The receiving UE can transmit the CSI as the feedback signal. Thecontents of the CSI include, for example, CQI (Channel Quality Indicatorindicating reception quality of received data), RI (rank index), and soforth. For example, the receiving UE calculates the CSI by using thereceived DM-RS (or a new RS).

Note that, in the D2D, transmission power and the number of times ofRepetitions can be changed, so that Link adaptation can be performedwith a dynamic range that is wider than the dynamic range of the CQIaccording to related art. Thus, a new CQI that can take more numericalvalues may be defined for the D2D.

In this manner, feedback of reception quality (SINR) can be performed ina wider range than that of the existing CQI; however, it can beconsidered that the overheads for the CSI feedback are increased.

Thus, the receiving UE in the embodiment can report, as the CSIfeedback, relative (rough) reception quality information with respect toData transmitted from the transmitting UE.

The rough reception quality is, for example, the margin of the receivedSINR (the difference from the required SINR), the number of times ofcombined reception with which the Data can be received, and so forth;however, it is not limited to these, and it can be any other informationas long as it is reception quality information with a reducedinformation amount. Furthermore, as an example of transmitting roughreception quality, reception quality may be implicitly reported by atransmission power control command.

Furthermore, as an example of transmitting the rough reception quality,ACK/NACK may be reported for each Data Repetition transmission(including the initial transmission). The receiving side of the ACK/NACK(the transmitting UE) can roughly estimate the reception quality by theratio of the ACKs/NACKs among a plurality of Data transmissions.

As described above, by transmitting the CSI with the feedback signal,Link adaptation with a high convergence property can be achievedcompared to Link adaptation based only on the ACK/NACK, while keepingthe low probability of occurrence of block errors. Furthermore, adynamic range that is wider than that of the existing CQI report can besupported.

Here, the receiving UE may use the above-described relative (rough) CSIonly for reporting it together with ACK/NACK; and may report theabsolute CSI in another format when the transmitting UE requests theCSI. In this manner, by allowing to report the absolute CSI, convergenceof Link adaptation can be expedited.

With Regard to Bundling (Bundling)

As described above, the receiving UE can bundle the ACKs/NACKs forrespective MAC PDUs of Data received from the transmitting UE to reportit to the transmitting UE. There are various methods for bundlingitself; however, for example, there is a bundling method such that onlyNACKs are transmitted without transmitting ACKs.

The receiving UE may always apply bundling, or may apply bundling if thenumber of the Data transmission subframes reported through the SA fromthe transmitting UE is greater than or equal to a predetermined value.The predetermined value is a value such that, when the number of theData transmission subframes exceeds the predetermined value, the amountof the feedback resources exceeds the amount of the resources that canbe used for transmitting the feedback signals.

An example of the bundling is illustrated by referring to FIGS. 12Athrough 12C. FIG. 12A shows, as a reference, a case where bundling isnot performed. FIG. 12B is an example where consecutive several (FIG.12B shows a case of two) ACKs/NACKs are bundled, so that the ACK/NACKsequence is (almost) uniformly bundled. FIG. 12C is an example ofapplying bundling to the tail of the ACK/NACK sequence.

By applying bundling in this manner, the overheads of the ACK/NACKfeedback can be reduced. Since the D2D Data is assumed to operate at alow BLER, deterioration of the throughput by bundling is considered tobe limited.

Example of ACK/NACK, CSI Mapping

Next, an example of mapping of feedback information according to theembodiment is described. Mapping of the feedback information may be madeby a signal point constellation; may be code division multiplexed (CDM)and reported as a sequence; or may be performed by defining a pluralityof feedback resources (channels) and reported by channel selection.

A time and frequency resource may be the resource to be a target of theabove-described channel selection; or a Cyclic shift and an OCC sequencemay be used separated from the time and frequency resource, or inaddition to the time and frequency resource.

For example, channel 1 (a resource defined as the channel 1)transmission may be associated with the NACK transmission; channel 2 (aresource defined as the channel 2) transmission may be associated withthe ACK transmission; and reception quality information (e.g., roughCSI) may be transmitted on each channel.

FIG. 13 illustrates the example of this case. FIG. 13 illustrates theexample where the SIR margin is transmitted as the reception qualityinformation. Furthermore, for each of the ACK and NACK, asymmetricmapping of the quality information is made. Here, one ACK/NACK and onereception quality are multiplexed; however, when multiple ACKs/NACKs areto be fed back, the averaged reception quality information may bemultiplexed with the ACKs/NACKs. Alternatively, only the ACK/NACK may betransmitted without combining the ACK/NACK and the reception qualityinformation.

As described above, by transmitting the ACK/NACK by the channelselection (resource selection), for example, in Group-cast, even ifACKs/NACKs from multiple receiving UEs are multiplexed, the transmittingUE (the UE receiving the feedback) can detect the ACK/NACK of each UE bydetermining the power.

Furthermore, since, for the UE transmitting the NACK, the SINR is lowand the channel estimation accuracy is unfavorable, by making theasymmetric mapping of the quality information as shown in FIG. 13,reporting of the rough quality information during transmission of theNACK can be more reliably executed.

With Regard to the Feedback Using the SCI

Here, in particular, a detailed example is described for a case wherethe feedback SCI is used, as described in the resource configurationexample 2.

When the receiving UE performs feedback signal transmission by using thefeedback SCI, the feedback signal transmission may be formed by defininga new SCI payload length that is different from that of the existing SA.As a result, a payload length that is different from that of the usualSCI (SA) can be supported, and efficient feedback can be performed.

Furthermore, when the receiving UE performs feedback signal transmissionby using the feedback SCI, the feedback may be formed with the payloadlength that is the same as that of the existing SA. In this case, thereceiving UE (the UE transmitting the feedback) differentiates thefeedback SCI from the existing SCI (SA) by a part of bits in the SCI. Asan example, in the feedback SCI, a part of bits is set to apredetermined setting value (e.g., a setting value that is not used inthe usual D2D). The transmitting UE (the UE receiving the feedback) canidentify the feedback SCI by the value of the bit.

For example, since 64 QAM is not used in the D2D, when the 64 QAM isreported as the MCS in the SCI, it is regarded as the feedback SCI.Alternatively, the CRC may be masked with a predetermined bit stream forthe feedback SCI, and if the mask can be removed by the predeterminedbit stream at the transmitting UE (the side of receiving the feedback),a determination may be made that it is the feedback SCI. Alternatively,the feedback SCI may be identified by the RS signal (e.g., DMRS cyclicshift). In this manner, by using the feedback SCI having the payloadlength that is the same as that of the existing SCI, it is possible toidentify whether the SCI is for feedback without increasing the numberof times of blind detection.

With Regard to the Scheduling SCI

Here, a detailed example of the scheduling SCI corresponding to the SAis described. As exemplified in FIG. 14, in the scheduling SCI thatsupports retransmission etc. caused by the feedback, it is desirable toassociate the resource to be retransmitted with Data to beretransmitted. In the following, a detailed example including amechanism for doing this is described.

In the embodiment, as an example, a new SCI format is defined that isdifferent from the format of the existing SA; and all the Contentsinformation items described in the quotation marks below, or acombination of any two or more of these information items is allowed tobe reported from the transmitting UE to the receiving UE. Additionally,information that is not shown below may be transmitted.

Contents: “existing SCI contents+a retransmission index/identifier, aNDI, the number of times of Data Repetitions, a CSI request, a feedbacktransmission resource indicator, the number of transmission MAC PDUs,and a TPC command”

The existing SCI contents are a TA, a L1 ID, a MCS, a frequencyresource, a time resource (T-RPT index), and so forth.

The retransmission index/identifier is information indicating that theretransmission is for which Data, when the Data to be transmitted is theretransmission Data. With this information, the receiving UE canidentify the target of retransmission.

The NDI (New Data Indicator) is information indicating that Data is fornew transmission; and, with this, retransmission in the SA cycle can besupported. The number of times of Data repetitions indicates the numberof times of Repetitions of the Data; and, by adjusting this number, abalance can be obtained between the effective throughput and thereception quality.

The CSI request is information instructing CSI transmission to thereceiving UE; and, with this, the transmitting UE can obtain the CSIinformation in an aperiodic manner. Furthermore, the CSI request may bea request for transmitting a signal to be used for the CSI measurement.

The feedback transmission resource indicator (e.g., ARI: ACK/NACKresource indicator) is information for allocating (specifying) afeedback transmission resource to the receiving UE; and, with this,collision among the feedback transmission resources can be avoided.

Note that, for the feedback resource allocation from the transmitting UEto the receiving UE, Clustered PUSCH transmission or full bandwidthtransmission may be reported, as an example.

The number of transmission MAC PDUs is the number of the MAC PDUs to beused for Data transmission by the transmitting UE; and, with this, whenthe number of the MAC PDUs that can transmit Data is greater than thenumber of the transmission MAC PDUs, the receiving UE can avoidunnecessary reception attempts. Additionally, with this, the receivingUE can differentiate a subframe that is not transmitted due to absenceof the transmission Data from a subframe in which Data reception erroroccurs. Furthermore, mixed scheduling of retransmission and newtransmission can be achieved, which is described below. For a case wherethe number of the transmission MAC PDUs is not reported by thescheduling SCI, the transmitting UE may transmit a signal with a specialformat in a subframe having no transmission Data. In this manner, thereceiving UE can determine that there is no transmission Data.

The TPC command is a command for adjusting the transmission power of thereceiving UE (the feedback transmitting side); and, with this, a balancecan be obtained between the reception quality and an interfering levelby adjusting the transmission power of the Unicast feedback.

As an example, transmission may be performed using 2PRB as a format ofnew scheduling SCI. This value is greater than that of the existing SCI(SA). As a result, even if the SCI payload length is increased, thequality can be ensured. Furthermore, for each SA resource pool thatarrives periodically, the SCI format that can be transmitted may belimited. For example, it can be considered to transmit new schedulingSCI and the existing SA at different cycles, respectively. In thismanner, the number of times of blind detection can be suppressed, andcollisions among the SCI formats can be avoided.

Note that the scheduling SCI for Data retransmission may be definedseparately from the scheduling SCI for new transmission; and thetransmitting UE may perform the SCI transmission in accordance with thedefinition. In this case, the receiving UE that has transmitted the NACKcan preferentially detect the scheduling SCI for retransmission within arange of the number of times of blind detection, so that it is effectivefor reducing the number of times of the blind detection, and forreducing the length of the SCI payload.

With Regard to the Scheduling SCI: Detailed Example of theRetransmission Index

Next, a detailed example of the above-described retransmission index isdescribed. For example, the transmitting UE reports, as theretransmission index, an index of the scheduling SCI of Data to beretransmitted, or a resource index of the Data.

Furthermore, for example, the transmitting UE may report all of or apart of the parameters that are used for determining the transmissionresource of the scheduling SCI for the Data to be retransmitted; or mayreport an identifier that has been reported to the receiving UE throughthe scheduling SCI. Alternatively, any of or all of the transmissionresource index, the Data resource allocation information, and the T-RPTindex of the above-described scheduling SCI may be reported.Additionally, an ID of the transmitting UE may be reported. By reportingthe above-described information, when Data is received from multipleUEs, the receiving UE can identify the transmission sources tosynthesize it.

Furthermore, the transmitting UE may report, as the retransmissionindex, the SA cycle of the Data to be retransmitted (at which point oftime (the point of time in units of cycles), the Data is transmitted),so that retransmission in a range from a predetermined number of cycles(N) before to that point of time is allowed. Namely, the transmitting UEperforms retransmission of an amount corresponding to up to N cycles;and the receiving UE also expects retransmission of the amountcorresponding to up to N cycles.

Furthermore, retransmission may be performed using a resource based onthe transmission resource index of the initial scheduling, withouttransmitting the retransmission index. For example, the retransmissionmay be performed by using a resource that is the same as that of theinitial transmission; or a transmission resource may be determined byperforming deterministic hopping among SA cycles. Namely, the hopping isdefined, so that a resource of the retransmission Data can be determinedfrom the resource of the initial transmission Data; and the transmittingUE and the receiving UE perform transmission and reception of theretransmission Data in accordance with the hopping rules. In thismanner, by avoiding to use the retransmission index, signaling overheadscan be reduced.

With Regard to the Scheduling SCI: An Example of SelectiveRetransmission

The transmitting UE may report, to the receiving UE, an index of, forexample, the MAC PDU to be retransmitted, as the retransmission index;and may perform Data retransmission of selective subframes (or MACPDUs). Specifically, for example, for Unicast, when NACK is received,the transmitting UE transmits the scheduling SCI including an index ofthe transmission MAC PDU corresponding to the NACK; and retransmits theData corresponding to the transmission MAC PDU. Furthermore, forexample, when NACK is received, the transmitting UE transmits thescheduling SCI including an index of the transmission subframecorresponding to the NACK; and retransmits the Data corresponding to thetransmission subframe. In the receiving UE, it is possible to identifywhich part of Data is retransmitted by the index, so that Data can beassembled. With such a configuration, retransmission can be implementedwith fewer subframes compared to the initial transmission.

Furthermore, in order to prevent feedback reception errors, thetransmitting UE may report, to the receiving UE by the scheduling SCI,the number of the MAC PDUs for which NACKs (or ACKs) are reported, or aconfirmation bit stream corresponding to the number of the MAC PDUs; andthe receiving UE may transmit feedback corresponding to the number.

With Regard to Scheduling SCI: Mixed Scheduling of Retransmission andNew Transmission

In the embodiment, the transmitting UE may perform, for the receivingUE, mixed scheduling of retransmission and new transmission.

For example, the transmitting UE performs mixed scheduling ofretransmission and new transmission by reporting, to the receiving UE bythe scheduling SCI, the number of the retransmission/new transmissionMAC PDUs. As an example, when retransmission target Data and newtransmission data exist, if the number of the MAC PDUs to beretransmitted is less than the number of the MAC PDUs that can betransmitted, the transmitting UE transmits, to the receiving UE by thescheduling SCI, scheduling information of the retransmission and the newtransmission and the NDI for the new transmission; and time-multiplexesthe retransmission MAC PDU and the new MAC PDU to transmit them. Anexample of this case is shown in FIG. 15 as example 1. In this case,after the retransmission, the new MAC PDU is to be transmitted.

Furthermore, as another example (example 2 in FIG. 15), the transmittingUE may perform mixed scheduling by transmitting the scheduling SCI forthe retransmission and the scheduling SCI for the new transmission. Morespecifically, for example, “the TA, the L1 ID, the T-RPT index, and thefrequency resource” are made the same and the NDI is made differentbetween the scheduling SCI for the retransmission and the scheduling SCIfor the new transmission, so that the new MAC PDU is transmitted afterthe retransmission MAC PDU.

Note that the feedback may be common feedback (example X of FIG. 15)regardless of the retransmission MAC PDU or the new MAC PDU; or feedbackmay be made independently for the retransmission and the new one(example Y of FIG. 15).

Device Configuration of the First Embodiment: User Equipment

FIG. 16 shows a functional configuration diagram of the user equipmentUE according to the embodiment. The UE shown in FIG. 16 is a UE that canbe any one of the transmitting UE and the receiving UE described of thefirst embodiment. As illustrated in FIG. 16, the user equipment UEincludes a signal transmitter 101; a signal receiver 102; a D2Dcommunication functional unit 103; a scheduling SCI transmissioncontroller 104; a feedback controller 105; and a retransmissioncontroller 106. Note that FIG. 16 only illustrates, in the userequipment UE, the functional units particularly related to theembodiment of the present invention; and functions, which are notdepicted, for performing at least operation conforming to LTE are alsoincluded. Additionally, the functional configuration illustrated in FIG.16 is merely an example. The functional division and the names of thefunctional units may be any division and names, as long as the operationof the UE according to the embodiment can be executed.

The signal transmitter 101 includes a function for generating varioustypes of physical layer signals from higher layer signals to betransmitted from the user equipment UE, and for wirelessly transmittingthem. Additionally, the signal transmitter 101 has a transmissionfunction of the D2D communication; and a transmission function of thecellular communication.

The signal receiver 102 includes a function for wirelessly receivingvarious types of signals from any other user equipment UE or the basestation eNB, and for retrieving the higher layer signal from thereceived physical layer signals. The signal receiver 102 has a receptionfunction of the D2D communication; and a reception function of thecellular communication.

The D2D communication functional unit 103 includes a D2D applicationfunction; and executes Discovery signal transmission and receptioncontrol, and SA/Data transmission and reception control. The schedulingSCI transmission controller 104 executes processing for transmission ofthe scheduling SCI (SA), which is described in the first embodiment. Asan example, the scheduling SCI transmission controller 104 creates asignal of the scheduling SCI; and performs signal mapping, etc. of thescheduling SCI for the signal transmitter 101.

The feedback controller 105 executes processing (including transmissionand reception of the scheduling SCI) for feedback transmission, which isdescribed in the first embodiment. Additionally, the retransmissioncontroller 106 executes processing for retransmission, which isdescribed in the first embodiment.

Second Embodiment

Next, the second embodiment is described. As described above, the secondembodiment relates to a technique of retransmission during Mode 1resource allocation. Detailed subject matter is as follows. Note thatthe second embodiment is based on the first embodiment; however, thesecond embodiment can be separately implemented.

ACK/NACK is transmitted from the receiving UE to the transmitting UE;however, in Mode 1 resource allocation, D2D transmission resourceallocation is performed by signaling from the base station eNB to theUE. Thus, even if the transmitting UE receives the NACK, theretransmission resource may not be immediately allocated from the basestation eNB to the transmitting UE, so that it can be considered thatretransmission latency occurs by waiting for the resource allocation.

Accordingly, in this embodiment, the above-described problem is solvedby introducing the following processing procedure. As an example of theprocessing procedure, processing procedure examples 2-1 through 2-3 aredescribed below.

Processing Procedure Example 2-1

The processing procedure example 2-1 is described by referring to FIG.17. At step 101, the transmitting UE makes a dynamic D2D schedulingrequest to the base station eNB. The D2D scheduling request may be arequest for retransmission; or a request for new transmission. Here, forexample, by making the scheduling request by L1 (e.g., PUCCH) signaling,dynamic transmission request can be implemented for the newtransmission/retransmission. Additionally, a SA resource pool index thatis to be the target of the request for allocation, an index of the Datato be retransmitted, and so forth may be included in the D2D schedulingrequest.

The base station eNB that receives the D2D scheduling request performsresource allocation (step 102). Then, the transmitting UE performsretransmission (or new transmission) by using the allocated resource(step S103).

In the processing procedure example 2-1, for example, by limiting thetransmission of the scheduling request (step 101) only for theretransmission, the overheads can be reduced.

Processing Procedure Example 2-2

The processing procedure example 2-2 is described by referring to FIG.18. Suppose that the transmitting UE transmits Data, and receives NACKfrom the receiving UE (steps 201, 202). The transmitting UE reports theNACK to the base station eNB (step 203). Note that, if ACK is received,the ACK may be reported. For example, by reporting NACK by L1 (e.g.,PUCCH), dynamic transmission resource allocation can be implemented.

The base station eNB that receives NACK determines that resourceallocation for retransmission is required, and performs, for thetransmitting UE, resource allocation for retransmission (step 204); andthe transmitting UE performs retransmission by using the resource (step205).

Processing Procedure Example 2-3

The processing procedure example 2-3 is described by referring to FIG.19. In the processing procedure example 2-3, NACK transmitted from thereceiving UE is received by the base station eNB (step 302). Thetransmitting UE does not receive the NACK, and does not transmit NACK tothe base station eNB. The transmitting UE determines retransmission Databased on the resource allocation at step 303, and performsretransmission at step 304. Note that the transmitting UE may receiveNACK at step 302, and may determine the retransmission Data based on theNACK.

In the processing procedure example 2-3, since the base station eNBdetermines the necessity of retransmission by receiving the feedback ofthe D2D, the feedback operation in Mode 1 may be limited to a case whereboth the transmitting and receiving UEs are RRC_CONNECTED or within thecoverage. For other cases, the operation is based on Mode 2 resourceallocation.

In the processing procedure example 2-3, by matching, for the UEs, thetransmission timing of feedback, such as NACK, with UL timing (uplinktransmission timing), reception of the feedback by the base station eNBcan be facilitated. In order to ensure reception by the base stationeNB, the transmission power may be adjusted to be the same as or a valueobtained by applying an offset value to that of PUCCH or PUSCH.

Other Detailed Examples

In the second embodiment, when scheduling (resource allocation) isperformed from the base station eNB to the UE by Mode 1 scheduling,Contents information described in the quotation marks below or acombination of any two or more of them may be reported by DCI.

Contents: “existing contents for Mode 1 scheduling+a retransmissionindex, a NDI, the number of times of Data Repetitions, a request for CSIreport, and a feedback transmission resource indicator.”

Among the above-described information items, the existing contents forMode 1 scheduling are Hopping flag, Data RB allocation, T-RPT index, SAresource index, and so forth. The information items other than theexisting contents for Mode 1 scheduling are as described above. Thetransmitting UE can perform retransmission by transmitting thescheduling SCI including these information items to the receiving UE.

In the second embodiment, the transmitting UE may report, to the basestation eNB, the CSI received as a feedback signal from the receivingUE. The reporting may be performed by higher layer signaling, or L1signaling. By finding the CSI from the receiving UE, the base stationeNB can determine the resource amount to be allocated to thetransmission buffer for the transmitting US based on the CSI. Forexample, if the reception quality is favorable, a large resource amountcan be allocated.

Furthermore, the transmitting UE may report the result of the Linkadaptation with the receiving UE to the base station eNB. The result ofthe Link adaptation is, for example, the MCS, the number of Repetitions,the transmission power, the transmission bandwidth, and so forth. Thereporting may be performed by higher layer signaling, or may beperformed by L1 signaling. By finding the result of the Link adaptationwith the receiving UE, the base station eNB can determine the resourceamount to be allocated to the transmission buffer for the transmittingUE based on the result of the Link adaptation. For example, ifhigh-speed transmission is possible, a large resource amount can beallocated.

Device Configuration of the Second Embodiment: User Equipment

FIG. 20 shows a functional configuration diagram of the user equipmentUE according to the embodiment. The UE illustrated in FIG. 20 is a UEthat can be any one of the transmitting UE and the receiving UEdescribed in the second embodiment. Additionally, the UE illustrated inFIG. 20 may include a function for implementing the operation of the UEdescribed in the first embodiment, and/or a function for implementingthe operation of the UE described in the third embodiment.

As illustrated in FIG. 20, the user equipment UE includes a signaltransmitter 201; a signal receiver 202; a D2D communication functionalunit 203; a scheduling SCI transmission controller 204; a feedbackcontroller 205; and a retransmission controller 206. Note that FIG. 20only illustrates, in the user equipment UE, the functional unitsparticularly related to the embodiment of the present invention; andfunctions, which are not depicted, for performing at least operationconforming to LTE are also included. Additionally, the functionalconfiguration illustrated in FIG. 20 is merely an example. Thefunctional division and the names of the functional units may be anydivision and names, as long as the operation of the UE according to theembodiment can be executed.

The signal transmitter 201 includes a function for generating varioustypes of physical layer signals from higher layer signals to betransmitted from the user equipment UE, and for wirelessly transmittingthem. Additionally, the signal transmitter 201 has a transmissionfunction of the D2D communication; and a transmission function of thecellular communication.

The signal receiver 202 includes a function for wirelessly receivingvarious types of signals from any other user equipment UE or the basestation eNB, and for retrieving the higher layer signal from thereceived physical layer signals. The signal receiver 202 has a receptionfunction of the D2D communication; and a reception function of thecellular communication.

The D2D communication functional unit 203 includes a D2D applicationfunction; and executes Discovery signal transmission and receptioncontrol, and SA/Data transmission and reception control. The schedulingSCI transmission controller 204 executes processing for transmission ofthe scheduling SCI (SA) (which includes reception of the resourceallocation), which is used in the second embodiment. As an example, thescheduling SCI transmission controller 204 creates a signal of thescheduling SCI; and performs signal mapping, etc. of the scheduling SCIfor the signal transmitter 201.

The feedback controller 205 executes processing (including transmissionand reception of the scheduling SCI) for feedback transmission, which isdescribed in the second embodiment. Additionally, the retransmissioncontroller 206 executes processing for retransmission, which isdescribed in the second embodiment.

Device Configuration of the Second Embodiment: Base Station

FIG. 21 shows a functional configuration diagram of the base station eNBaccording to the embodiment. Note that the eNB illustrated in FIG. 21may include a function for implementing the operation of the eNBdescribed in the third embodiment. As shown in FIG. 21, the base stationeNB includes a signal transmitter 301; a signal receiver 302; a UEinformation storage unit 303; a D2D resource information storage unit304; and a scheduler 305. Note that FIG. 21 only illustrates, in thebase station eNB, the functional units particularly related to theembodiment of the present invention; and functions, which are notdepicted, for performing at least operation as a base station in thecommunication system conforming to LTE are also included. Additionally,the functional configuration illustrated in FIG. 21 is merely anexample. The functional division and the names of the functional unitsmay be any division and names, as long as the operation according to theembodiment can be executed.

The signal transmitter 301 includes a function for generating varioustypes of physical layer signals from higher layer signals to betransmitted from the base station eNB, and for wirelessly transmittingthem. The signal receiver 302 includes a function for wirelesslyreceiving various types of signals from user equipment UE, and forretrieving the higher layer signal from the received physical layersignals.

The UE information storage unit 303 stores information on UEcapabilities received from respective UEs. The D2D resource informationstorage unit 304 stores, for each UE, information indicating theallocated D2D resource. Furthermore, when the resource is released, theallocation information is deleted. As described by referring to FIG. 17through FIG. 19, the scheduler 305 has a function for performingresource allocation based on a request and feedback.

Third Embodiment

Next, the third embodiment is described. As described above, the thirdembodiment relates to feedback setting and feedback type switching, anddetails are as follows. Note that the third embodiment can be executedin combination with the first embodiment and/or the second embodiment.

Considering UEs (terminals) according to Rel-12 of LTE, it is not alwaystrue that all the UEs support the feedback in D2D. Furthermore, thenecessity of feedback differs depending on Broadcast/groupcast/unicast,etc.

Accordingly, in the embodiment, for example, for each resource pool, thepresence/absence of feedback and the feedback type (the resourceconfiguration examples 1 through 3, etc. of the first embodiment, or maybe the presence or absence of bundling, etc.) are configurable.Furthermore, feedback types corresponding to communication types(Broadcast/groupcast/unicast) may be defined in advance; and each UE maydetermine (set) the feedback type depending on the communication type tobe used. Note that the third embodiment is based on the first embodiment(and/or the second embodiment); however, the third embodiment can beseparately implemented.

Processing Procedure 3-1

FIG. 22 illustrates an example of a procedure of a feedback setting(Feedback configuration) by the base station eNB in Mode 1.

In this case, the transmitting UE requests the feedback setting from thebase station eNB (step 401). The request may be a request for feedbackresource allocation. Based on the request, the base station eNB reportsthe feedback setting to the UE by signaling. As illustrated in B throughD of FIG. 22, as a method of signaling the feedback setting, there arevarious types of methods. For example, as illustrated in B, the feedbacksetting may be reported from the base station eNB to each UE; or, asillustrated in C and D, the feedback setting may be reported from thetransmitting UE to the receiving UE.

For reporting the feedback setting from the transmitting UE to thereceiving UE, the SCI can be used. For example, the feedback settinginformation is included in the SCI.

When, at step 407, the feedback operation is enabled, for example, asshown in E (steps 408 and 409) and F (step 411), resource allocation forfeedback is performed for the receiving UE, and the feedback isperformed (step 410 and 412).

Processing Procedure 3-2

FIG. 23 shows an example of the procedure of feedback setting (Feedbackconfiguration) by the base station eNB in Mode 2.

In Mode 2, the feedback setting may be reported from the base stationeNB to each UE by the system information (SIB) (the process indicated bystep 501 of A); or the feedback setting may be reported from thetransmitting UE to the receiving UE (the process indicated by step 502of B).

When, at step 503, the feedback operation is enabled, for example, asillustrated in C (step 504), resource allocation for feedback isperformed for the receiving UE, and the feedback is performed (step505).

By the processing procedure examples 3-1 and 3-2, the feedback settingcan be shared among the transmitting UE, the receiving UE, and the eNB,so that proper feedback operation can be implemented.

Note that, as exemplified in F of FIG. 22 and D of FIG. 23, bytransmitting resource allocation or a feedback request to the receivingUE, the base station eNB may cause the receiving UE to transmit afeedback signal (e.g., the CSI), and the base station eNB may receivethe feedback signal.

Another Detailed Example

In the third embodiment, the receiving UE may switch the feedback format(synonymous with feedback type) of the feedback signal, depending on theconfiguration of the resource pool to be used, the number of thetransmission subframes of Data, and so forth. As an example, thereceiving UE can switch the presence/absence of ACK/NACK bundling,depending on the maximum number of the transmission subframes of Data(the number of the subframes that can be transmitted) that can betransmitted from the receiving UE. For example, the receiving UE (the UEtransmitting the feedback) can make a determination such that, when themaximum number of the transmission subframes is less than apredetermined number, the ACK/NACK bundling is applied; and, when themaximum number of the transmission subframes is greater than or equal tothe predetermined number, the ACK/NACK bundling is not applied. In thismanner, the overheads due to feedback can be optimized, depending on thesubframes of Data that can be transmitted.

Furthermore, the receiving UE may determine whether to report only theACK/NACK, or to report it while including the reception quality,depending on the request for the CSI received from the transmitting UE.For example, even if the CSI request is received, if it is determinedthat the CSI report is unnecessary (example: a case where apredetermined time has not elapsed from the previous CSI transmission),only the ACK/NACK may be reported. In this manner, when the CSI reportis unnecessary, the detection accuracy of ACK/NACK in the transmittingUE (the UE receiving the feedback) can be enhanced.

Furthermore, the receiving UE may determine whether the ACK/NACKfeedback and the CSI feedback are to be independently reported,depending on the request for the CSI received from the transmitting UE.For example, if it is determined that it is necessary to feed backaccurate CSI (example: a case where Link adaptation is not properlyexecuted), the ACK/NACK feedback and the CSI feedback may beindependently reported. If the CSI is to be independently reported, highprecision CSI reporting is allowed.

Furthermore, the receiving UE may use different feedback formats inUnicast/group-cast, respectively. For example, in Groupcast, theACKs/NACKs that are bundled based on the PUCCH format 1 may betransmitted; and, in Unicast, the high precision ACK/NACK may betransmitted based on the PUCCH format 3.

Furthermore, with respect to the feedback in Group-cast, the feedbackformat that can be used during Group-cast may be limited to a specificformat (example: a format capable of transmitting only ACK/NACK as thefeedback), so that only the ACK/NACK can be determined by thetransmitting UE (the UE receiving the feedback). Furthermore, inGroup-cast, the receiving UE may return the feedback only for a case ofNACK. Through such a contrivance, even for a response in which feedbacksignals from multiple receiving UEs are multiplexed, the transmitting UEof Group-cast can determine whether ACK is reported or NACK is reported.

Furthermore, with respect to the feedback in Group-cast, by performingfeedback by multiple UEs with the same resource, increase ininterference with an adjacent frequency resource can be avoided.Specifically, for example, multiple feedback resources may be defined,and the receiving UE (the UE transmitting the feedback) may performtransmission of the feedback by randomly selecting a resource.Furthermore, in order to avoid increase in interference, thetransmitting UE that detects the interference, or the base station eNBthat detects the interference may prohibit the receiving UE fromperforming the feedback in Group-cast. Furthermore, the receiving UE maynot autonomously perform the feedback in Group-cast.

Device Configuration of the Third Embodiment: User Equipment

FIG. 24 shows a functional configuration diagram of the user equipmentUE according to the embodiment. The UE illustrated in FIG. 24 is a UEthat can be any one of the transmitting UE and the receiving UEdescribed in the third embodiment. Additionally, the UE illustrated inFIG. 24 may include a function for implementing the operation of the UEdescribed in the first embodiment, and/or a function for implementingthe operation of the UE described in the second embodiment.

As illustrated in FIG. 24, the user equipment UE includes a signaltransmitter 401; a signal receiver 402; a D2D communication functionalunit 403; a scheduling SCI transmission controller 404; a feedbackcontroller 405; and a retransmission controller 406. Note that FIG. 24only illustrates, in the user equipment UE, the functional unitsparticularly related to the embodiment of the present invention; andfunctions, which are not depicted, for performing at least operationsconforming to LTE are also included. Additionally, the functionalconfiguration illustrated in FIG. 24 is merely an example. Thefunctional division and the names of the functional units may be anydivision and names, as long as the operation of the UE according to theembodiment can be executed.

The signal transmitter 401 includes a function for generating varioustypes of physical layer signals from higher layer signals to betransmitted from the user equipment UE, and for wirelessly transmittingthem. Additionally, the signal transmitter 401 has a transmissionfunction of the D2D communication; and a transmission function of thecellular communication.

The signal receiver 402 includes a function for wirelessly receivingvarious types of signals from any other user equipment UE or the basestation eNB, and for retrieving the higher layer signal from thereceived physical layer signals. The signal receiver 202 has a receptionfunction of the D2D communication; and a reception function of thecellular communication.

The D2D communication functional unit 403 includes a D2D applicationfunction; and executes Discovery signal transmission and receptioncontrol, and SA/Data transmission and reception control. The schedulingSCI transmission controller 404 executes processing for transmission ofthe scheduling SCI (SA) (which includes reception of the resourceallocation), which is used in the third embodiment for retransmission,resource allocation, transmission of the feedback setting, and so forth.As an example, the scheduling SCI transmission controller 404 creates asignal of the scheduling SCI; and performs signal mapping, etc. of thescheduling SCI for the signal transmitter 401.

The feedback controller 405 executes processing (including transmissionand reception of the scheduling SCI) for feedback transmission, which isdescribed in the third embodiment. The feedback controller 405 includesa function for receiving the feedback setting, and for setting(configure) the UE based on the setting information. The retransmissioncontroller 406 executes processing for retransmission.

Device Configuration of the Third Embodiment: Base Station

FIG. 25 shows a functional configuration diagram of the base station eNBaccording to the embodiment. Note that the eNB illustrated in FIG. 25may include a function for implementing the operation of the eNBdescribed in the second embodiment. As shown in FIG. 25, the basestation eNB includes a signal transmitter 501; a signal receiver 502; aUE information storage unit 503; a D2D resource information storage unit504; a scheduler 505; and a feedback setting controller 506. Note thatFIG. 25 only illustrates, in the base station eNB, the functional unitsparticularly related to the embodiment of the present invention; andfunctions, which are not depicted, for performing at least operations asa base station in the communication system conforming to LTE are alsoincluded. Additionally, the functional configuration illustrated in FIG.25 is merely an example. The functional division and the names of thefunctional units may be any division and names, as long as theoperations according to the embodiment can be executed.

The signal transmitter 501 includes a function for generating varioustypes of physical layer signals from higher layer signals to betransmitted from the base station eNB, and for wirelessly transmittingthem. The signal receiver 502 includes a function for wirelesslyreceiving various types of signals from user equipment UE, and forretrieving the higher layer signal from the received physical layersignals.

The UE information storage unit 503 stores information on UEcapabilities received from respective UEs. The D2D resource informationstorage unit 504 stores, for each UE, information indicating theallocated D2D resource. Furthermore, when the resource is released, theallocation information is deleted. The scheduler 505 has a function ofperforming resource allocation based on a request and feedback. Thefeedback setting controller 506 includes a function of performingfeedback setting according to the present embodiment.

HW Configuration Examples in First Through Third Embodiments

The configurations of the user equipment UEs respectively illustrated inFIG. 16, FIG. 20, and FIG. 24 may be entirely implemented by a hardwarecircuit (example: one or more IC chips); or a part may be implemented bya hardware circuit.

FIG. 26 is a diagram illustrating an example of a hardware (HW)configuration (the configuration that is common among the first throughthird embodiment) of the user equipment UE. FIG. 26 shows aconfiguration that is closer to an implementation example compared tothe configurations illustrated in FIG. 16, FIG. 20, and FIG. 24. Asillustrated in FIG. 26, the UE includes a RE (Radio Equipment) module651 for performing processing related to radio signals; a BB (Base Band)processing module 652 for performing baseband signal processing; adevice control module 653 for performing higher layer processing, etc.;and a USIM slot 654 that is an interface for accessing a USIM card.

The RE module 651 applies D/A (Digital-to-Analog) conversion,modulation, frequency conversion, power amplification, etc. to a digitalbaseband signal received from the BB processing module 652 to generate aradio signal to be transmitted from an antenna. Furthermore, by applyingfrequency conversion, A/D (Analog to Digital) conversion, modulation,etc. to the received radio signal, a digital baseband signal isgenerated, and it is passed to the BB processing module 652. The REmodule 651 includes, for example, functions of the physical layer, etc.for the signal transmitter (101, 201, 401) and the signal receiver (102,202, 401).

The BB processing module 652 performs a process of mutually convertingthe IP packet and the digital baseband signal. A DSP (Digital SignalProcessor) 662 is a processor that performs signal processing in the BBprocessing module 652. The memory 672 is used as a work area of the DSP662. The BB processing module 652 includes, for example, functions ofthe layer 2, etc. for the signal transmitter (101, 201, 401) and thesignal receiver (102, 202, 402); the D2D communication functional unit(103, 203, 403); the scheduling SCI transmission controller (104, 204,404); the feedback controller (105, 205, 405); and retransmissioncontroller (106, 206, 406). Note that all of or a part of the D2Dcommunication functional unit (103, 203, 403), the scheduling SCItransmission controller (104, 204, 404), the feedback controller (105,205, 405), and retransmission controller (106, 206, 406) may be includedin the device control module 653.

The device control module 653 performs IP layer protocol processing,various application processing, and so forth. The processor 663 is aprocessor that performs processing that is executed by the devicecontrol module 653. The memory 673 is used as a work area of theprocessor 663. Additionally, the processor 663 performs reading out datafrom and writing data in the USIM through the USIM slot 154. Theconfiguration of the base station eNB illustrated in FIG. 21 and FIG. 25may be implemented entirely by a hardware circuit (example: one or moreIC chips); or a part of which may be formed of a hardware circuit, andthe other part may be implemented by a CPU and a program.

FIG. 27 is a diagram illustrating an example of a hardware (HW)configuration (the configuration that is common among the first throughthird embodiment) of the base station eNB. FIG. 27 shows a configurationthat is closer to an implementation example compared to theconfigurations illustrated in FIG. 21 and FIG. 25. As illustrated inFIG. 27, the base station eNB includes a RE module 751 for performingprocessing related to radio signals; a BB (Base Band) processing module752 for performing baseband signal processing; a device control module753 for performing higher layer processing, etc.; and a communication IF754 that is an interface for connecting to a network.

The RE module 751 applies D/A conversion, modulation, frequencyconversion, power amplification, etc. to a digital baseband signalreceived from the BB processing module 752 to generate a radio signal tobe transmitted from an antenna. Furthermore, by applying frequencyconversion, A/D conversion, modulation, etc. to the received radiosignal, a digital baseband signal is generated, and it is passed to theBB processing module 752. The RE module 751 includes, for example,functions of the physical layer, etc. for the signal transmitter (301,501) and the signal receiver (302, 502).

The BB processing module 752 performs a process of mutually convertingthe IP packets and the digital baseband signals. A DSP (Digital SignalProcessor) 762 is a processor that performs signal processing in the BBprocessing module 752. The memory 772 is used as a work area of the DSP762. The BB processing module 752 includes, for example, functions ofthe layer 2, etc. for the signal transmitter (301, 501) and the signalreceiver (302, 502); the UE information storage unit (303, 503); the D2Dresource information storage unit (304, 504); the scheduler (305, 505);and the feedback setting controller 506. Note that all of or a part ofthe UE information storage unit (303, 503);the D2D resource informationstorage unit (304, 504); the scheduler (305, 505); and the feedbacksetting controller 506 may be included in the device control module 753.

The device control module 753 performs IP layer protocol processing, OAMprocessing, and so forth. The processor 763 is a processor that performsprocessing that is executed by the device control module 753. The memory773 is used as a work area of the processor 763. The auxiliary storagedevice 783 is, for example, a HDD, etc.; and stores various types ofsetting information and so forth for the base station eNB itself tooperate.

Conclusion of the Embodiments

As described above, according to the embodiments (the first to thirdembodiments, the same applies hereinafter), there is provided userequipment to be used as receiving user equipment in a mobilecommunication system supporting D2D communication, the user equipmentincluding a feedback unit that receives a D2D signal from transmittinguser equipment, and that transmits, to the transmitting user equipment,a feedback signal with respect to the D2D signal by using apredetermined resource; and a receiver that receives a retransmissionD2D signal transmitted from the transmitting user equipment based on thefeedback signal.

By the above-described configuration, in the mobile communication systemsupporting the D2D communication, a technique is provided that allowsfeedback and retransmission to be performed between the user equipmentand the user equipment.

The feedback unit determines the predetermined resource based on, forexample, resource information explicitly or implicitly reported from thetransmitting user equipment. With such a configuration, a resource forfeedback transmission can be appropriately determined.

The feedback unit may determine the predetermined resource based onscheduling control information corresponding to the D2D signal, thescheduling control information being received from the transmitting userequipment. With such a configuration, the resource for feedbacktransmission can be determined by the scheduling control informationcorresponding to the D2D signal, which can be a target ofretransmission, so that the resource of the feedback signalcorresponding to the D2D signal can be quickly determined.

The feedback unit may use, as the predetermined resource, a resource inan SA resource pool or a resource in a resource pool for D2D datatransmission. By using the resource in the SA resource pool, thefeedback signal can be transmitted as control information; and by usingthe resource in the resource pool for the D2D data transmission, thefeedback signal can be transmitted as data.

The receiver may receive the scheduling control information forreporting a transmission resource of a retransmission D2D signal, priorto receiving the retransmission D2D signal, and the scheduling controlinformation may include information indicating the D2D signal to beretransmitted. With such a configuration, the user equipment can findthat the retransmission is to be performed for which D2D signal.

Furthermore, according to the embodiments, there is provided userequipment to be used as transmitting user equipment in a mobilecommunication system supporting D2D communication, the user equipmentincluding a receiver that receives, from receiving user equipment, afeedback signal with respect to a D2D signal transmitted to thereceiving user equipment; and a transmitter that transmits, to thereceiving user equipment, a retransmission D2D signal with respect tothe D2D signal, based on the feedback signal.

By the above-described configuration, in the mobile communication systemsupporting the D2D communication, a technique is provided that allowsfeedback and retransmission to be performed between the user equipmentand the user equipment.

The user equipment may include a control information transmitter thattransmits, to the receiving user equipment, scheduling controlinformation including transmission resource information of the D2Dsignal, and the scheduling control information may include resourceinformation to be used for determining a resource for transmitting thefeedback signal in the receiving user equipment. With such aconfiguration, the resource for transmitting the feedback signal can beproperly determined in the receiving user equipment.

Furthermore, the control information transmitter may transmit, to thereceiving user equipment, the scheduling control information includinginformation indicating the D2D signal to be retransmitted, prior to theretransmission D2D signal being transmitted. With such a configuration,the receiving user equipment can find that retransmission is to beperformed for which D2D signal.

Furthermore, in the embodiments, there is provided user equipmentincluding both the units of the above-described transmitting userequipment and the units of the receiving user equipment.

Note that the “unit” in the configuration of each of the above-describeddevices may be replaced with “part,” “circuit,” “device,” and so forth.

The user equipment UE described in the embodiments may have aconfiguration such that, it includes a CPU and a memory, and it isimplemented by executing a program by the CPU (a processor); aconfiguration that is implemented by hardware, such as a hardwarecircuit, including logic for performing the process described in theembodiments; or a configuration in which programs and hardware coexist.

The base station eNB described in the embodiments may have aconfiguration such that, it includes a CPU and a memory, and it isimplemented by executing a program by the CPU (a processor); aconfiguration that is implemented by hardware, such as a hardwarecircuit, including logic for performing the process described in theembodiments; or a configuration in which programs and hardware coexist.

The embodiments of the present invention are described above; howeverthe disclosed invention is not limited to the embodiments, and a personordinarily skilled in the art will appreciate various variations,modifications, alternatives, replacements, and so forth. Specificexamples of numerical values are used in the description in order tofacilitate understanding of the invention; however, these numericalvalues are merely an example, and any other appropriate values may beused, except as indicated otherwise. The separations of the items in theabove description are not essential to the present invention; anddepending on necessity, subject matter described in two or more itemsmay be combined and used, and subject matter described in an item may beapplied to subject matter described in another item (provided that theydo not contradict). A boundary of a functional unit or a processor inthe functional block diagrams may not necessarily correspond to aboundary of a physical component. An operation by a plurality offunctional units may be physically executed by a single component, or anoperation of a single functional unit may be physically executed by aplurality of components. For the convenience of description, the userequipment UE and the base station eNB are described by using thefunctional block diagrams; however, such devices may be implemented inhardware, software, or combinations thereof. Each of the software to beexecuted by the processor included in the user equipment UE inaccordance with the embodiment of the present invention and the softwareto be executed by the processor included in the base station eNB inaccordance with the embodiment of the present invention may be stored inany appropriate storage medium, such as a random access memory (RAM), aflash memory, a read-only memory (ROM), an EPROM, an EEPROM, a register,a hard disk drive (HDD), a removable disk, a CD-ROM, a database, aserver, and so forth.

The present invention is not limited to the above-described embodiment;and various variations, modifications, alternatives, replacements, andso forth are included in the present invention without departing fromthe spirit of the present invention.

This patent application is based upon and claims the benefit of priorityof Japanese Patent Application No. 2014-232135 filed Nov. 14, 2014, theentire contents of Japanese Patent Application No. 2014-232135 areincorporated herein by reference.

LIST OF REFERENCE SYMBOLS

-   eNB: base station-   UE: user equipment-   101: signal transmitter-   102: signal receiver-   103: D2D communication functional unit-   104: scheduling SCI transmission controller-   105: feedback controller-   106: retransmission controller-   201: signal transmitter-   202: signal receiver-   203: D2D communication functional unit-   204: scheduling SCI transmission controller-   205: feedback controller-   206: retransmission controller-   301: signal transmitter-   302: signal receiver-   303: UE information storage unit-   304: D2D resource information storage unit-   305: scheduler-   401: signal transmitter-   402: signal receiver-   403: D2D communication functional unit-   404: scheduling SCI transmission controller-   405: feedback controller-   406: retransmission controller-   501: signal transmitter-   502: signal receiver-   503: UE information storage unit-   504: D2D resource information storage unit-   505: scheduler-   506: feedback setting controller-   651: RE module-   652: BB processing module-   653: device control module-   654: USIM slot-   751: RE module-   752: BB processing module-   753: device control module-   754: communication IF

1. A user equipment to be used as a receiving user equipment in a mobilecommunication system supporting Device-to-Device (D2D) communication,the user equipment comprising: a feedback unit that receives a D2Dsignal from a transmitting user equipment, and that transmits, to thetransmitting user equipment, a feedback signal with respect to the D2Dsignal by using a predetermined resource; and a receiver that receives aretransmission D2D signal transmitted from the transmitting userequipment based on the feedback signal.
 2. The user equipment accordingto claim 1, wherein the feedback unit determines the predeterminedresource based on, resource information explicitly or implicitlyreported from the transmitting user equipment.
 3. The user equipmentaccording to claim 1, wherein the feedback unit determines thepredetermined resource based on scheduling control informationcorresponding to the D2D signal, the scheduling control informationbeing received from the transmitting user equipment.
 4. The userequipment according to claim 1, wherein the feedback unit uses, as thepredetermined resource, a resource in a Scheduling Assignment (SA)resource pool, or a resource in a resource pool for D2D datatransmission.
 5. The user equipment according to claim 1, wherein thereceiver receives the scheduling control information for reporting atransmission resource of a retransmission D2D signal, prior to receivingthe retransmission D2D signal, and the scheduling control informationincludes information indicating the D2D signal to be retransmitted.
 6. Auser equipment to be used as a transmitting user equipment in a mobilecommunication system supporting Device-to-Device (D2D) communication,the user equipment comprising: a receiver that receives, from areceiving user equipment, a feedback signal with respect to a D2D signaltransmitted to the receiving user equipment; and a transmitter thattransmits, to the receiving user equipment, a retransmission D2D signalwith respect to the D2D signal, based on the feedback signal.
 7. Theuser equipment according to claim 6, further comprising: a controlinformation transmitter that transmits, to the receiving user equipment,scheduling control information including transmission resourceinformation of the D2D signal, wherein the scheduling controlinformation includes resource information to be used for determining aresource for transmitting the feedback signal in the receiving userequipment.
 8. The user equipment according to claim 7, wherein thecontrol information transmitter transmits, to the receiving userequipment, the scheduling control information including informationindicating the D2D signal to be retransmitted, prior to theretransmission D2D signal being transmitted.
 9. A feedback controlmethod to be executed by a user equipment that is used as a receivinguser equipment in a mobile communication system supportingDevice-to-Device (D2D) communication, the feedback method comprising: afeedback step of receiving a D2D signal from a transmitting userequipment, and transmitting, to the transmitting user equipment, afeedback signal with respect to the D2D signal by using a predeterminedresource; and a reception step of receiving a retransmission D2D signaltransmitted from the transmitting user equipment, based on the feedbacksignal.
 10. A retransmission control method to be executed by a userequipment that is used as a transmitting user equipment in a mobilecommunication system supporting Device-to-Device (D2D) communication,the retransmission control method comprising: a reception step ofreceiving, from a receiving user equipment, a feedback signal withrespect to a D2D signal transmitted to the receiving user equipment; anda transmission step of transmitting, to the receiving user equipment, aretransmission D2D signal with respect to the D2D signal, based on thefeedback signal.
 11. The user equipment according to claim 2, whereinthe feedback unit determines the predetermined resource based onscheduling control information corresponding to the D2D signal, thescheduling control information being received from the transmitting userequipment.
 12. The user equipment according to claim 2, wherein thefeedback unit uses, as the predetermined resource, a resource in an SAresource pool, or a resource in a resource pool for D2D datatransmission.
 13. The user equipment according to claim 3, wherein thefeedback unit uses, as the predetermined resource, a resource in an SAresource pool, or a resource in a resource pool for D2D datatransmission.
 14. The user equipment according to claim 11, wherein thefeedback unit uses, as the predetermined resource, a resource in an SAresource pool, or a resource in a resource pool for D2D datatransmission.
 15. The user equipment according to claim 2, wherein thereceiver receives the scheduling control information for reporting atransmission resource of a retransmission D2D signal, prior to receivingthe retransmission D2D signal, and the scheduling control informationincludes information indicating the D2D signal to be retransmitted. 16.The user equipment according to claim 3, wherein the receiver receivesthe scheduling control information for reporting a transmission resourceof a retransmission D2D signal, prior to receiving the retransmissionD2D signal, and the scheduling control information includes informationindicating the D2D signal to be retransmitted.
 17. The user equipmentaccording to claim 4, wherein the receiver receives the schedulingcontrol information for reporting a transmission resource of aretransmission D2D signal, prior to receiving the retransmission D2Dsignal, and the scheduling control information includes informationindicating the D2D signal to be retransmitted.
 18. The user equipmentaccording to claim 11, wherein the receiver receives the schedulingcontrol information for reporting a transmission resource of aretransmission D2D signal, prior to receiving the retransmission D2Dsignal, and the scheduling control information includes informationindicating the D2D signal to be retransmitted.
 19. The user equipmentaccording to claim 12, wherein the receiver receives the schedulingcontrol information for reporting a transmission resource of aretransmission D2D signal, prior to receiving the retransmission D2Dsignal, and the scheduling control information includes informationindicating the D2D signal to be retransmitted.
 20. The user equipmentaccording to claim 13, wherein the receiver receives the schedulingcontrol information for reporting a transmission resource of aretransmission D2D signal, prior to receiving the retransmission D2Dsignal, and the scheduling control information includes informationindicating the D2D signal to be retransmitted.